How to animate with random values in Freestyle Lines
The stroke and gap lengths can be keyframed. If you, for example, set keyframes at the beginning and end of the animation range, at value 1 for them all, then, in the graph editor, you can give their f-curves a Noise modifier:

I think you're still limited to a (spatially) repeating cycle of 3 dash-and-gap pairs, but it might still do?